The short answer
Record the internal hole cross-section, the depth that will actually receive concrete and the real cross-section of the post within that concrete. If line, corner, end or gate-post holes differ, record them separately. Once those measurements are known, use the Post Hole Concrete Calculator to calculate net volume, whole bags and an editable ordering allowance.
Confirm the required hole before measuring
This guide explains how to measure a specified hole; it does not determine how deep or wide a post hole should be. Post-system instructions, project documents, soil and loading conditions, local requirements and qualified advice can all affect the required detail.
Before digging in the United States, contact 811 and wait for the applicable utility responses. The national 811 Before You Dig service explains the process and why even smaller projects such as fences and mailboxes require a locate request.
Measure a round hole
- Measure the actual internal diameter across the widest part of the concrete-filled hole.
- Measure the depth that will receive concrete, not automatically the full excavated depth.
- Record the number of holes that share those same dimensions.
Diameter is the full distance across the circle. Do not enter the radius unless a calculator explicitly asks for it.
Measure a rectangular hole
Measure the internal length, internal width and concrete-filled depth. If the sides flare or the excavation is visibly irregular, take representative measurements and keep any allowance visible rather than pretending the hole is a perfect prism.
Round hole area = π × (diameter ÷ 2)²
Rectangular hole area = internal length × internal width
Measure the buried post
For a round post, record its actual diameter. For a square or rectangular post, record its actual finished length and width. Product names and nominal lumber sizes should not be substituted automatically for a measured or manufacturer-stated finished cross-section.
Also record how far the post extends through the concrete-filled portion. If it passes through the complete concrete depth, that length equals the concrete-filled depth. If it stops partway, only the overlapping length displaces concrete.
Account for non-concrete layers
A gravel base, soil cap or other non-concrete layer reduces the concrete-filled depth only when it is part of the actual specified installation. Record the excavated depth and concrete-filled depth as different measurements rather than silently subtracting a generic layer.
Separate different post-hole groups
Do not average different hole sizes. Group only holes with matching shape, dimensions, post cross-section and concrete-filled depth. For example, line posts may form one group while larger corner, end or gate-post holes form separate groups.
This preserves the real geometry and lets the final allowance and whole-bag rounding happen once across the complete project.
Why the post is subtracted
The post occupies part of the hole, so concrete fills only the remaining space. The useful formula is the hole volume minus the post volume within the same concrete-filled depth.
Concrete per hole = hole area × concrete-filled depth − post area × post length within concrete
The calculator checks whether the selected post shape can physically fit inside the entered hole before subtracting displacement.
Common measuring errors
- Entering radius when the field asks for diameter.
- Using a nominal post description instead of actual finished dimensions.
- Counting the full excavation as concrete when another specified layer occupies part of the depth.
- Averaging visibly different line, corner, end or gate-post holes.
- Rounding each hole to whole bags instead of combining the project first.
Post-hole measurement FAQs
What dimensions do I measure for a round post hole?
Measure the actual internal diameter and the depth that will be occupied by concrete.
What dimensions do I measure for a rectangular hole?
Measure the internal length, internal width and concrete-filled depth.
Why is the post volume subtracted?
The post occupies part of the hole, so only the remaining space can contain concrete.
Should I use the nominal post size or its actual size?
Use the actual measured cross-section or the manufacturer's stated finished dimensions.
What if some holes are different sizes?
Record each matching set separately. Different corner, end, gate or line-post holes should not be hidden inside one average.
How deep and wide should my post hole be?
This guide does not decide that. Use the project or post-system specification, local requirements or qualified advice for hole design.
Sources and review status
- 811 Before You Dig: safe digging process
- QUIKRETE Concrete Mix Data Sheet 1101: product uses and published yields
- OSHA: controlling hazards in concrete products
Sources checked September 2, 2026. This guide has been checked against the cited primary sources; it has not been independently reviewed by an engineer, contractor or other named professional.
Measurements ready? Combine matching hole groups, subtract actual post displacement and round only the final purchase quantity.
Calculate Concrete from These Measurements